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XYLENE POWER LTD.

FNR INDICATOR TUBES

By Charles Rhodes, P.Eng., Ph.D.

INDICATOR TUBE PURPOSE:
The purpose of the indicator tubes is to transmit to the FNR control system the insertion distance of each movable fuel bundles into the fixed fuel bundle matrix, the discharge temperature of each movable fuel bundle and the gamma flux originating from each movable fuel bundle.
 

INDICATOR TUBE DESCRIPTION:
There are buoyant indicator tubes attached to the movable fuel bundles. These indicator tubes extend vertically 0.3 m to 1.5 m above the liquid sodium coolant surface. The indicator tubes are field attached to the movable fuel bundles. Each indicator tube consists of a central tube which is open at both ends and a surrounding concentric sealed buoyant gas filled annular region. The gas filled annular region ensures that the temperature of the liquid sodium in the central tube is approximately the same as the temperature of the liquid sodium at the discharge of the associated movable fuel bundle.

The annular region of the indicator tube provides sufficient positive buoyancy so that when 1.5 m of the indicator tube is projecting above the primary liquid sodium surface the indicator tube maintains a firm upright position.

Note that the buoyancy of the annular region is not sufficient to lift the net weight of a movable fuel bundle when the indictor tube is fully immersed in liquid sodium.

The gamma ray spectrum propagating up the annular region indicates the relative fission power in each movable fuel bundle.
 

THE FLOAT:
Each indicator tube contains a 6 m long float consisting of a sealed 6 m long thin wall steel tube (0.5 inch OD) nearly filled with lithium. The lithium is less dense than the sodium and prevents long term float penetration by liquid sodium. The average density of the float is less that the lowest possible density of the liquid sodium in the indicator tube center. The liquid sodium has a higher thermal coefficient of expansion than the steel. Hence the elevation difference between the top of the float and the liquid sodium coolant surface level in the indicator tube center varies with the liquid sodium temperature. Each float can be precisely weighted so that the temperature indicated by the float length above the liquid sodium surface is precise and is highly reproducible over time. This temperature measurement precision and reproducibility is an important aspect of power FNR safety.

In operation the liquid sodium level in the indicator tube core is approximately the same as the liquid sodium level Es outside the indicator tube. As the liquid sodium temperature in the indicator tube center rises the liquid sodium density in the indicator tube center decreases faster than does the average density of the float. Hence the elevation difference between the top of the float and the liquid sodium surface decreases with increasing liquid sodium temperature. That elevation difference, and hence the liquid sodium temperature, is measured with an overhead laser apparatus.
 

LASER TARGETS:
Each indicator tube presents three concentric round elevation targets to an overhead laser scanner. The central target, about 4 inches in diameter, at elevation Ea, is fixed to the top of the float. The middle target, about 8 inches in outside diameter, at elevation Eb is fixed to the top of the indicator tube wall. The lowest target, effectively about 11 inches in outside diameter, at elevation Es, is the surface of the coolant liquid sodium. The middle target outside diameter must be less than 11 inches to allow fixed fuel bundles to be selectively removed without disturbing the adjacent movable fuel bundles.

Note that when the movable fuel bundles are fully inserted into the matrix of fixed fuel bundles the indicator tube top projects about 1.5 m above the liquid sodium coolant surface. Hence the float must extend about 1.6 m above the liquid sodium coolant surface. In order to do so the float tube extends 4.4 m below the liquid sodium surface. A practical length for the float tube is 6 m.

The density of liquid Li is 0.512 g / cm^3. There is a plenum space above the Li to permit the Li to thermally expand and contract without changing the float mass or external float dimensions. However, the float diameter and length will increase with temperature causing a reduction in the average float density, but that reduction is smaller than the reduction in liquid sodium density with increasing temperature.

The vertical position of each movable fuel bundle is calculated from:
(Eb - Es)
which is in the range:
0.1 m to 1.3 m

Another device measures the elevation (Es - Et) where Et is the elevation of the top of the fixed fuel bundles.

For each indicator tube the vertical separation between the movable and fixed fuel bundles is given by:
[7.5 m - (Es - Et) - (Eb - Es)]

The temperature of the liquid sodium in the indicator tube core is calculated from its density which is a function of the float parameters and:
(Ea - Es)
 

INDICATOR TUBE ATTACHMENT:
Indicator tubes are attached to the movable fuel bundles after the movable fuel bundles are installed and are removed before the movable fuel bundles are relocated. The indicator tube attachment point is the movable fuel bundle lifting point. The indicator tubes must have dual J type bottom hooks for attachment to the lifting points of the movable fuel bundles.
 

LIFTING POINT:
A FNR fuel bundle lifting point is achieved by replacing the (3 / 16) inch thick diagonal plates with (3 / 8) inch thick diagonal plates in the upper portion of the fuel bundle where there are fuel tube plenums. The two (3 / 8) inch thick diagonal plates extend above the tops of the fuel tubes. Two 3.0 inch diameter holes in each plate form the lifting point.

The diagonal plates connecting each fuel bundle lifting point to the corresponding fuel bundle corner girders must also allow unobstructed primary liquid sodium flow and must not prevent individual fuel tube insertion or extraction.
 

INDICATOR TUBE MATERIAL AND DIMENSIONS:
The indicator tube diameter should be minimal to minimize obstruction of the natural liquid sodium circulation, but must be sufficient to allow accurate steady state movable fuel bundle liquid sodium discharge temperature measurement.

The indicator tubes are fabricated from HT-9 steel (85% Fe, 12% Cr, 1% Mo, 0% C, 0% Ni). The tube plus hook places the top of the indicator tube 7.5 m above the top 0f the movable fuel bundles. Hence the indicator tube itself is only about 7 m long.

The height allowances for the fixed fuel bundle components from bottom to top are: legs (1.5 m), bottom grating (0.1 m), fuel tubes (6 m), lifting point (0.3 m), swelling allowance 0.1 m. Hence the fuel bundle shipping container and the air lock tube must be able to accommodate a fuel bundle with an overall length of 8.0 m. This same air lock is long enough to accommosate the indicator tubes.
 

Indicator Tube: 4.0 inch OD X 0.____ inch wall X 7.5 m long_______
Mass = Pi X 4.0 inch X 0.____ inch X 7.5 m X (.0254 m / inch)^2 X 7.874 X 10^3 kg / m^3
= ____ kg
 

VERTICAL THERMAL EXPANSION:
Note that the open steel lattice near the bottom of the primary liquid sodium pool and the fuel bundles will thermally expand vertically with increasing surrounding liquid sodium temperature. During normal reactor operation the open steel lattice is likely to be about 120 degrees C cooler than the liquid sodium temperature at the top of the fuel bundle. The thermal expansion will be significant and will affect the calculation of the movable fuel bundle insertion into the matrix of fixed fuel bundles unless temperature compensating measurements are performed. Hence the overhead laser scanner needs a compensating fixed fuel bundle elevation measurement.

The differential vertical thermal expansion per fuel bundle is approximately:
20 ppm / deg C X 430 deg C X 16.5 m = 0.1419 m
Hence it is essential that the laser scanning system cancel out vertical thermal expansion.  

MOVABLE FUEL BUNDLE TRAVEL LIMIT:
The movable fuel bundle vertical travel is limited at the bottom by the probe length (1.2 m) and the hydraulic cylinder end piece and piston thickness (0.3 m) and height of the steel lattice (1.5 m) and at the top by a hydraulic actuator vent hole.

The extent of insertion of a mobile square fuel bundle into the fixed fuel bundle matrix is determined by the volume of liquid sodium inside the fuel bundle's hydraulic actuator. There is fluid pressure feedback which indicates the approximate mobile fuel bundle vertical position due to the changing buoyancy of the indicator tube. The hydraulic fluid feed tube is routed through the open steel lattice. This hydraulic tube must be sufficiently flexible to allow for +/- 1 m ______earthquake induced movement of the primary sodium pool with respect to its concrete enclosure.
 

This web page last updated May 29, 2021

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